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Concept
Dynamic soaring
Summary
Dynamic soaring is a flying technique used to gain energy by repeatedly crossing the boundary between air masses of different velocity. Such zones of wind gradient are generally found close to obstacles and close to the surface, so the technique is mainly of use to birds and operators of radio-controlled gliders, but glider pilots are sometimes able to soar dynamically in meteorological wind shears at higher altitudes.
Dynamic soaring is sometimes confused with slope soaring which is a technique for achieving elevation.
While different flight patterns can be employed in dynamic soaring, the simplest is a closed loop across the shear layer between two airmasses in relative movement, e.g. stationary air in a valley, and a layer of wind above the valley. The gain in speed can be explained in terms of airspeed and groundspeed:
As the glider begins the loop, say in a stationary airmass, groundspeed and airspeed are the same.
The glider enters the moving airmass nearly head-on, which increases the glider's airspeed.
The glider then turns 180°, where it is able to maintain most of its airspeed due to momentum. This must happen immediately, or groundspeed will be lost. The glider's groundspeed, first crosswind, then downwind, as it turns, is now higher, as the tailwind has accelerated the glider.
The loop continues with the glider re-entering the stationary airmass and turning around, maintaining the now higher airspeed and groundspeed.
Each cycle results in higher speeds, up to a point where drag prevents additional increase.
The energy is extracted by using the velocity difference between the two airmasses to lift the flying object to a higher altitude (or to reverse the descent respectively) after the transfer between the airmasses.
In practice, there is a turbulent mixing layer between the moving and stationary air mass. In addition, drag forces are continually slowing the plane. Since higher speed gives rise to higher drag forces, there is a maximum speed that can be attained. Typically around 10 times the windspeed for efficient glider designs.
Official source
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